Brass union in wall?

I decided to install a brass union below the hot washin g machine faucet to make it possible to remove/replace in the future. It is a 1/2" sweat connection on one side and 1/2" threaded on the other. I ended up using two wraps of teflon tape over the brass seat to keep it from leaking (I know tape is not suppose to be used but nothing else seemed to work to stop it from leaking and the pipes were aligned straight). Should I be concerned about closing this union in the wall behind sheetrock? After three days, there are no leaks. Is it common for unions to loosen up over time and start leaking from water hammer or vibrations?

I fail to understand how the union would simplify removal if it is buried behind drywall. In a previous shower install, the valves had integrated unions and one of them started to leak a year later. I had to cut the drywall on the other side of the wall to get to it.

I agree with LLigetfa. If you have a problem with the union leaking, and I have some concerns with a union that didn't seat/seal without tape anyway, you will have to open the wall to make the repair and probably change the union. A sweat joint would be fair less likely to fail and on the off chance that it did, you could heat and make the repair much easier. Dry wall repair is one of the easiest repairs you can make in a home, so I wouldn't waste a nickle try something fancy to possibly avoid it.

I fail to understand how the union would simplify removal if it is buried behind drywall. In a previous shower install, the valves had integrated unions and one of them started to leak a year later. I had to cut the drywall on the other side of the wall to get to it.

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I should have explained it better. I bought an Oakley washing maching box. The left valve installs with a quarter turn to lock it in the plastic box. The only way to remove it without a union is to cut the copper pipe and re-solder. I wanted to have the option to replace it without resoldering it in the wall. The union allows me to just cut and repair the sheetrock or perhaps install an access panel below the box. I used an old (unused) brass union that my dad had laying around from the 70s. It seemed to be much better than the new ones found today at the box stores. I still have the fittings and pipe to sweat connect it directly if the union is going to be risky.

BTW, the middle cold valve (right hand drain box) has a plastic nut below and can be unscrewed without a union.

The threads on washing machine boxes are "straight threads" for the fastening lock nut. They do NOT seal to pipe fittings properly since to do that the thread would have to be tapered.[/QUOTE
The threads on the hot are tapered since it is molded into a quarter turn plastic connector (the taper is obvious and its only threads about 1"). However you may be right about the middle cold. It tightened like it was tapered but did have a lock ring and was threaded all the way up. How do you usually install these types? BTW, No leaks yet after day four.

IF I were to use a threaded brass fitting on those threads, I would solder the two pieces together, however, that would negate the "easy to remove" feature you were trying to achieve. A "straight thread" WILL eventually bottom out in a tapered fitting, but it is only sealing by that last thread.The rest of them are "loose".

I should have explained it better. I bought an Oakley washing maching box..

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Are you sure it isn't an Oatey? Do you have a model number for it? It may remove all doubt about the threads versus sweat connections. Personally, I would go with sweat connections even considering re-entering it later.